This invention is generally directed to a an improved layered organic photoresponsive device, and more specifically the present invention is directed to the utilization of specific materials in a layered organic photoresponsive device, which device is suitable for color highlight imaging systems, especially the single step reproduction of color images, particularly black and red images. In accordance with one aspect of the present invention, there is thus provided an improved layered organic photoresponsive device containing therein certain materials for each of the layers, thereby allowing such a device to have optimal spectral response, excellent photosensitivity, and stable cycling properties.
Two color reproduction systems serving distinct needs, are generally known, namely representational color wherein the colors of the reproduction match those of the original document; and functional color, wherein the color to be reproduced merely serves to mark distinguish or highlight portions of a document such as a text, graphs, or line drawings. In representational color systems, images are xerographically produced for example by three successive color filter exposures, followed by an in register transfer of toner images produced by three toners of the appropriate primary attractive colors. Such systems are complex in that they require the superimposition of images on three separate exposures, either in three successive cycles, or on a photoreceptor of sufficient circumference or length to accomodate three successive images prior to transfer. Also it is known to use in such systems a series of three separate in-register photoreceptor drums each contributing one image to the final transfer sheet, however, such a system is costly, and can result in images of poor resolution in view of the complexity of the system and the necessity for having three separate photoreceptor drums. In the simpler functional color imaging systems generally only two colors need to be reproduced, although more than two colors can be produced if desired. In one such system, to which the present invention relates there is produced two color functional color documents wherein for example black may be used to represent the main text, and red or blue those selected portions of the text, figures and the like, which portions are to be directed to the users special attention by means of highlight color. In such systems accordingly, there can be produced images in two colors, such as red and black, by desirably employing only one imaging operation. In many instances full color copying is not desired since for example the documents being copied, such as accounting documents and other business documents contain colors of black and red only, in addition to a white background. The photoreceptor device of the present invention can be utilized to produce clored image copies of such documents. Illustrative examples of other documents that may be subjected to the highlight color process of the present invention include technical journals such as Scientific American, a large portion of whose pages are printed in black and highlight color; engineering drawings, letters, reports, and a variety of other documents created by color ink, crayon, signature impression stamp, typewriter ribbon and the like.
There is described in U.S. Pat. No. 4,188,213, issued on Feb. 12, 1980, a method for producing color copies involving a number of complex steps including for example the recording of successive single color electrostatic latent images on an image bearing member, followed by developing each successive color electrostatic latent image with particles containing a predetermined dominant colorant therein corresponding to each recorded single color electrostatic latent image, transferring layers of the developed particles to a sheet of support material, and regulating electrically the transfer step in order that successive thiner layers of particles are transferred from the image bearing member to the sheet of support material. It is disclosed in this patent that development in one embodiment involves the deposition of particles containing a dominant cyan colorant with a minor magenta colorant impurity, on an electrostatic latent image formed from a red filtered image, depositing particles containing a dominant magenta colorant with a minor yellow impurity on the electrostatic latent image formed from a green filtered light image, and depositing particles containing a dominant yellow colorant on the electrostatic latent image formed from a blue filtered light image. Each successive layer of toner particles which are transferred to a sheet of support material, contains a color, corresponding in color, to the color of impurity contained in the previously transferred layer of toner particles. Thus, successive layers of toner particles are transferred in superimposed registration with one another, with each successive transferred layer of toner particles correcting for the impurities contained in the colorant of the previously transferred layer of toner particles, thus producing a combination of toner particles substantially approximating the desired color.
U.S. Pat. No. 4,189,224 solves some of the problems of the process of the '213 patent in that the method described therein requires only a single exposure to derive a two color image, therefore registration and multiple cycling steps are eliminated. More specifically, there is described in U.S. Pat. No. 4,189,224 a two color electrostatic copying apparatus which can be operable for one color positive or negative copying. In accordance with the teachings of this patent, a photoconductive material containing a conductive substrate, an inner photoconductive layer sensitive to visible light, and an outer photoconductive layer insensitive to red light, is subjected to an electrostatic charge, which charge is applied to the outer layer, while at the same time irradiating the device with light so as to render one of the layers conductive. Subsequently, an electrostatic charge of opposite polarity is applied to the outer layer of the photoresponsive member, this step being accomplished in the dark. A light image of an original document is then projected onto the outer layer of the photoresponsive device, wherein white areas of the image cause photoconduction of both layers and red areas thereof, causing photoconduction of only the inner layer, accordingly, as a result, white areas of the material have zero surface potential, while red and black areas have non-zero surface potentials of opposite polarities. The images can then be developed by employing for example red and black toner particles of opposite charge. Thus, for example, red particles which are charged positively will be caused to adhere to negatively charged image areas contained in the photoresponsive device, while black toner particles which are charged negatively will adhere to the black image areas which are charged positively. While the photoresponsive device described in this patent has its advantages, there continues to be a need for photoresponsive devices, particularly improved layered devices which would be useful in a highlight color imaging system, and have advantages in terms of cost, flexibility, choice of spectral response, configuration, versatility, and the like. Further there continues to be a need for layered organic photoresponsive devices possessing the capability of discriminating between two preselected separate input color regions, that is, for example, black and red print regions, versus white background, which devices may be employed to convert black and/or color image input into potential patterns. The patterns can then be developed by a number of known means including the method as described in U.S. Pat. No. 4,078,929, the disclosure of which is totally incorporated herein by reference.
Also, U.S. Pat. No. 4,078,929 discloses a single step electrostatographic copying process in which two different potential levels on a photoresponsive device may be developed in immediate sequence subsequent to a single exposure, by means of two differentially colored xerographic toners. The two potential levels may be of the same polarity or preferably of opposite polarities. In one embodiment of the invention disclosed in the '929 patent, positively charged toner particles of a first color and negatively charged toner particles of a second color are about evenly concentrated in the relatively negative and relatively positive areas of the imaging surface, thus allowing the positively charged toner particles to be attracted to the imaging surface which contains a negative charge pattern, while the negatively charged toner particles of a second color are attracted to the imaging surface which contains a positive charge pattern.